The invention concerns an end stop device for an electrically operated window screening arrangement of the type whose movement in one direction between two end positions corresponding to a screened window and an unscreened window, respectively, is effected by winding draw cords or strings connected with the screening arrangement on at least one winding drum positioned in a horizontal main frame or sash member of the window, said drum being over a motor shaft connected with an electric drive motor, whereas movement in the opposite direction is effected by means of a constant force during simultaneous unwinding of said draw cord or string from the winding drum, which end stop device comprises a contact for disruption of the motor current during movement in said opposite direction by activation from a sensor, the drive motor and said contact and sensor being positioned in or at said main frame or sash member.
DK publication no. 158 054 B discloses an electric drive mechanism for a venetian blind where the electric drive motor is provided on or built into the horizontal top section of the window sash. By hoisting the venetian blind, the motor is disconnected when the bottom rail of the venetian blind stops in the top position by detecting the motor current and activating a reset circuit when the power exceeds a pre-determined value.
For obtaining an end stop when lowering the venetian blind, the disconnection of the power supply to the motor in this known design is effected when the bottom rail reaches its bottom position in the way that a reed contact mounted on the bottom section of the sash is affected by a magnet positioned on the bottom rail of the venetian blind.
In addition to the practical inconvenience consisting in that an electrical wire connection has to be established between the reed contact at or in the sash bottom section and the motor arrangement in the top section of the sash, it is in the known drive mechanism not possible to bring the venetian blind to an automatic stop in the middle position during lowering if the bottom rail meets an obstacle, eg. a pot plant.
Furthermore, this known drive mechanism is not suited for other forms of screening devices as eg. roller blinds where it is the rolling down movement which is motor-driven, whereas the rolling up movement is effected by means of a constant spring force.
From DK publication no. 144 894 B, an electric operating device for a venetian blind is known where limit contacts for stopping the drive motor at both upward and downward movement are mounted in the motor casing built into the top list of the venetian blind, operating means for the contacts being journalled and guided in the motor casing and comprising a pin which protrudes through the motor casing and functions as an abutment for the bottom rail of the venetian blind in the top position and to which a projecting sensor lever is fastened, said lever is affected by the draw cord or string of the venetian blind and activates the contact when the cord or string are slackened at the stop of the bottom rail in the bottom position.
Although this embodiment represents a simplification as to installation, it requires a relatively complicated contact device and is aesthetically less attractive due to the operating members projecting from the motor casing.